Thermal inkjet printers have gained wide acceptance. These printers are described by W. J. Lloyd and H. T. Taub in "Ink Jet Devices," Chapter 13 of Output Hardcopy Devices (Ed. R. C. Durbeck and S. Sherr, San Diego: Academic Press, 1988 ) and U.S. Pat. Nos. 4,490,728 and 4,313,684. Thermal inkjet printers produce high quality print, are compact and portable, and print quickly and quietly because only ink strikes the paper.
An inkjet printer forms a printed image by printing a pattern of individual dots at particular locations of an array defined for the printing medium. The locations are conveniently visualized as being small dots in a rectilinear array. The locations are sometimes "dot locations", "dot positions", or pixels". Thus, the printing operation can be viewed as the filling of a pattern of dot locations with dots of ink. Inkjet printers print dots by ejecting very small drops of ink onto the print medium, and typically include a movable carriage that supports one or more printheads each having ink ejecting nozzles. The carriage traverses over the surface of the print medium, and the nozzles are controlled to eject drops of ink at appropriate times pursuant to command of a microcomputer or other controller, wherein the timing of the application of the ink drops is intended to correspond to the pattern of pixels of the image being printed.
Color thermal inkjet printers commonly employ a plurality of printheads, for example four, mounted in the print carriage to produce different colors. Each printhead contains ink of a different color, with the commonly used colors being cyan, magenta, yellow, and black. These base colors are produced by depositing a drop of the required color onto a dot location, while secondary or shaded colors are formed by depositing multiple drops of different base color inks onto the same dot location, with the overprinting of two or more base colors producing secondary colors according to well established optical principles.
The typical thermal inkier printhead (i.e., the silicon substrate, structures built on the substrate, and connections to the substrate) uses liquid ink (i.e., colorants dissolved or dispersed in a solvent). It has an array of precisely formed nozzles attached to a printhead substrate that incorporates an array of firing chambers which receive liquid ink from the ink reservoir. Each chamber has a thin-film resistor, known as a thermal inkjet firing chamber resistor, located opposite the nozzle so ink can collect between it and the nozzle. When electric printing pulses heat the thermal inkjet firing chamber resistor, a small portion of the ink next to it vaporizes and ejects a drop of ink from the printhead. Properly arranged nozzles form a dot matrix pattern. Properly sequencing the operation of each nozzle causes characters or images to be printed upon the paper as the printhead moves past the paper.
Print quality is one of the most important considerations of competition in the color inkjet printer field. Since the image output of a color inkjet printer is formed of thousands of individual ink drops, the quality of the image is ultimately dependent upon the quality of each ink drop and the arrangement of the ink drops on the print medium.
Plotters and printers that use thermal inkjet printheads are normally limited to operating in a mode that delivers a single drop volume. When printing halftone images or when a larger color gamut is required, artificial methods of simulating continuous tone printing are used such as multiple nozzles, multiple drops, pre-cursor heating, dithering and other digital halftoning methods. Most of these methods have serious drawbacks and design limitations.
Varying the drop volume can cause variations in the darkness of black-and-white text, variations in the contrast of gray-scale images, and variations in the chroma, hue, and lightness of color images. The chroma, hue, and lightness of a printed color depends on the volume of all the primary color drops that create the printed color. Controlling the drop volume improves the quality of printed text, graphics, and images.
The drop volume from an inkjet printhead can be adjusted by using the following factors: (1) the drop generation geometry (resistor physical size and exit orifice size), (2) the forces affecting the refill speed such as backpressure, filter resistance and entrance channel restrictions, (3) factors affecting the size and strength of the drive bubble such as ink temperature, the boiling surface heating rate and boiling surface cleanliness, and (4) effects on fluidic response such as the ink viscosity which is a function of the ink temperature.
The above factors can be divided into two categories: (1) factors that the printer can dynamically change and (2) factors that are fixed design parameters. Of the above factors only printhead temperature and the boiling surface heating rate (associated with the pulse width) can be dynamically adjusted by the printer.
Other methods have been used to vary the delivered ink drop volume. The first method is to vary the temperature of the printhead to change the ink viscosity and increase ejection efficiency, but this places increased stress on the printhead substrate and increases the likelihood of chemical interaction of the ink with the printhead substrate. This results in decreased chemical resistance of the printhead. The second method is to use more than one drop of ink per pixel location on the media, but this has an attendant decrease in the printer's throughput.
Thus, major advantages would be obtained if a simple and inexpensive method was available to vary the ink drop volume of an thermal inkjet printhead. These advantages include being able to produce improved halftone images in an inkjet printer.